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WASHINGTON, DC (RNN) - A potentially harmful and threatening letter was sent to President Barack Obama on Thursday, a day after it was revealed New York City Mayor Michael Bloomberg was mailed a letter laced containing ricin.

The letter addressed to Obama was intercepted before it reached the White House, and experts determined are analyzing it to determine if it contains the potentially deadly substance ricin. The letter will be tested at Fort Detrick in Maryland, where the U.S. Army Medical Command is located, and will probably take a week for definitive tests.

The FBI says the letter was sent from Spokane, WA, on May 13, the same day other letters with ricin were mailed from the same area.

Matthew Ryan Buquet was arrested on May 22 and charged with one count of mailing threatening communications for allegedly sending a threatening letter containing ricin to a Washington-area district court judge.

Two other letters that were intercepted were sent to a Washington-area post office and an air force base and are now part of the investigation into letters sent to Obama. Investigators determined a letter was also sent to the CIA but has not been found.

The four letters obtained by police - to Obama, the judge, the air force base and the post office - were addressed in red ink.

Meanwhile, in a separate case, the FBI questioned a New Boston, TX, man in connection to another incident in which similar letters containing traces of ricin were mailed from Shreveport, LA, to New York Mayor Michael Bloomberg.

According to Raycom station KSLA, the FBI and local authorities Thursday questioned Nathaniel Richardson, an Army veteran who works at the Red River Army Depot, and his wife Thursday night in connection with the letters mailed from Shreveport. He was questioned at his workplace and later at the home where he and his wife live with his five children. KSLA reported that agents removed electronics from the home.

Since the Shreveport postal center handles mail from Louisiana, Texas and Arkansas, the letters could have originated from any of those states, KSLA reports.

The content and message from those letters are similar to those sent to Obama.

The writer said in the letter to Bloomberg, "you'll have to kill me and my family before you get my guns," and that the right to bear arms is a constitutional right, according to information CNN obtained from law enforcement and Secret Service sources. The writer said he'll "exercise that right until the day I die."

Ricin is a poison that is present in castor beans, according the New York State Department of Health. It is part of the waste that is produced during the production of castor oil and as such, is one of the most easily produced plant toxins.

It's the second ricin letter sent to the president this year. Obama and Sen. Roger Wicker, R-MS, both received letters that tested positive for ricin on April 17.

Charges were dropped against Paul Kevin Curtis, who was first suspected of sending the letters to them.

Everett Dutschke, 41, of Tupelo, MS, was arrested April 27 following the search of his home and martial arts studio earlier. He has been charged with making and possessing ricin for use as a weapon, according to The Associated Press.

Dutschke faces up to life in prison if convicted. He's also facing unrelated charges of child molestation.

The letters that were sent to Obama and Wicker, were reported to have said, "To see a wrong and not expose it, is to become a silent partner to its continuance."